The flocculation of matter suspended in water to enhance clarification rates is an important aspect of industrial and municipal water treatment. Inorganic coagulants such as lime, alum, ferric chloride, ferrous sulfate, ferric sulfate and sodium aluminate have traditionally been used. These chemicals, while aiding in the removal of suspended solids, generally provide additional quantities of dissolved inorganic solids which can themselves provide water problems, particularly where the water is recirculated in substantial quantities.
Polymeric organic coagulants have also proved valuable for flocculation. These polyelectrolytes are often preferred because, in addition to their minimal affect on dissolved solids, they tend to produce less, more compact waste sludge, and they tend not to appreciably affect the pH of the treated water. Cationic polyamines are typical organic coagulants. In addition, high molecular weight polymers such as anionic polyacrylamides have been used to aid in flocculation achieved with primary organic or inorganic coagulants. Anionic polymers are used to aid settling and in conjunction with the aluminum or iron salts, while cationic polymers are preferably used in dewatering settled sludge and, in some cases, to aid in metal salt coagulation.
Tannins occur naturally in various vegetative materials including barks and woods. Established industrial practices of extracting tannins from the wood of the quebracho tree and the bark of the wattle tree have made condensed tannins available in substantial quantities. Condensed tannins are polyphenolic and polymerize in combination with other chemicals such as formaldehyde. A. Pizzi, in "Condensed Tannins for Adhesives" Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 359-369 discusses natural tannins and their particular use in the manufacture of particle board, plywood, and other industrial products which incorporate adhesives.
British Pat. No. 899,721 discloses use of a reaction product of a tannin, a formaldehyde and an amino or ammonium compound in the flocculation of suspensions such as sewage, industrial waste, and natural water. The disclosed advantages of using these flocculants are that they do not affect the pH of the suspension and that they do not affect the dissolved inorganic solids content of the treated water. A disadvantage of these products is that they tend to gel quickly, making them unfit for commerical use at locations remote from their production site.
U.S. Pat. No. 4,558,080 discloses the production of stable tannin-based flocculants by controlling the reaction of tannin with an aldehyde and an amino compound by, among other things, monitoring the viscosity of the reacting mixture. The resultant product has a shelf life which renders it commerically suitable as a flocculant and competitive with other well-known flocculants such as alum. However, as with other commercial flocculants, substantial quantities of the tannin-based flocculant must be used during water treatment. Means for increasing the efficiency of flocculation and clarification while reducing material consumption and other costs continue to be sought.